The thermodynamics and kinetics of calcite crystallization: Baseline for understanding biomineral formation

Calcite is the most abundant bio-mineral that forms in the nearly ubiquitous presence of aspartic acid-rich organic matrices. This study investigated the interactions of aspartic acid (Asp) with calcite surfaces and the kinetic and thermodynamic effect of Asp on calcite crystallization. The interactions of Asp with calcite {lcub}101¯4{rcub} faces were examined through dissolution experiments using in situ Fluid Cell AFM and ex situ optical methods. A change in etch pit morphologies on {lcub}101¯4{rcub} faces after the introduction of Asp suggests that the Asp-calcite crystal face interactions are direction-specific. The effect of Asp on calcite crystallization was examined by characterizing growth spirals in Asp-free and Asp-bearing solutions. In the absence of Asp, measured critical step length scaled linearly with 1/σ where σ is the supersaturation. This linear relationship confirmed the Gibbs-Thomson prediction and yielded the step edge free energy for calcite crystallization. After the introduction of Asp, a change in the morphology of growing spirals and a reduction of step edge free energies were observed. These findings demonstrate the possible energetic controls of Asp during calcite biomineralization.; Complex kinetic behavior was observed for step advancement in each direction in the absence and presence of Asp. The non-linear dependence of step velocity upon supersaturation was attributed to impurity effect and the kinetic coefficients of step advancement were obtained in Asp-free solutions based upon the models of impurity impact. A comparison of the kinetic data in Asp-free and Asp-bearing solutions suggests that Asp is not a growth inhibitor for calcite crystallization.; Rate expressions for spiral growth based upon surface processes and the theories of crystal growth were derived and compared with those based upon chemical affinities of precipitation reactions. An analysis of the relationship between the two types of rate laws demonstrated that affinity-based rate equations only approximate spiral growth under special conditions: growth proceeds by single soured, single spirals at near equilibrium supersaturations. This suggests that, in general, it is not appropriate to deduce growth mechanisms from macroscopic kinetic measurements.